Two step magnetron start circuit

ABSTRACT

A control arrangement for stepped energizing of a magnetron in a microwave oven includes means to delay the application of the full high voltage to the magnetron to insure operation in the desired oscillating mode. Such delay is accomplished by providing a thermistor coupled serially with an input terminal of a voltage doubler, such that upon magnetron conduction, the current flowing through the thermistor causes a decrease in resistance therein which in turn produces an increase in the doubler output voltage, whereupon the reaching of a predetermined level thereof, a relay effects switching to apply full line voltage to the voltage doubler input terminals.

[11] 3,821,594 June 28, 1974 [5.4] TWO-STEP MAGNETRON START CIRCUIT [75]Inventors: Robert F. Webb; John J. Roberts,

both of Louisville, Ky.

[73] Assignee: General Electric Company,

. Louisville, Ky.

[22] Filed: Mar. 28, 1973 21 Appl. No.: 345,678

[52] US. Cl. 315/104, 219/1055, 315/3951, 331/86 [51] Int. Cl. H01117/06 [58] Field of Search 219/1055; 315/102, 104; 331/86; 321/37 [56]References Cited 3/1972 Tingle-y 315/102 Primary Examiner-Herman KarlSaalbach Assistant Examiner-Richard A. Rosenberger Attorney, Agent, orFirm-Richard L. Caslin; Francis H. Boos, Jr.

[57] ABSTRACT A control arrangement for stepped energizing of amagnetron in a microwave oven includes means to delay the application ofthe full high voltage to the magnetron to insure operation in thedesired oscillating mode. Such delay is accomplished by providing athermistor coupled serially with an input terminal of a voltage doubler,such that upon magnetron conduction, the current flowing through thethermistor causes a decrease in resistance therein which in turnproduces an increase in the doubler output voltage, whereupon thereaching of a predetermined level thereof, a relay effects switching toapply full line voltage to the voltage doubler input terminals.

6 Claims, 1 Drawing Figure 1 TWO-STEP MAGNETRON START CIRCUIT BACKGROUNDOF THE INVENTION The present invention relates to a control arrangementfor a magnetron and more particularly to a twostep starting circuit fora domestic microwave oven magnetron, for protection of the magnetronagainst failure to oscillate such as often occurs when a magnetron isimmediately connected to a full wave high voltage supply, andparticularly when the high voltage is applied simultaneously withapplication of filament current.

Such failure to oscillate may be highly destructive to the magnetrontube. It is preferable that the cathode of a magnetron be heated to itsproper operating temperature before full anode voltage is applied. Ifthe anode voltage is applied before the cathode surface has beenuniformly and thoroughly heated to its operating temperature, onepossible result is that a very high density current flows from thecathode and application of the full anode voltage at this time subjectsthe cathode to the stress of a very strong electric field which maycause cathode material to be pulled from the cathode surface. Anotherpossible result is that the magnetron may operate improperly in a lowcurrent, high voltage mode which may eventually cause itsself-destruction. A delay then is necessary between the application ofthe cathode heater voltage and the application of the full anode voltageto allow the cathode emitting surface to reach its operatingtemperature.

One solution to the delay problem is to provide a timer motor to controlactivation of a relay having contacts arranged that, upon their closing,a currentlimiting series connected resistor will be by-passed, therebyallowing the full line voltage to be applied to the primary of atransformer supplying power to the mangetron. Such a solution is taughtin U.S. Pat. No. 3,392,309 Hickman. Another solution is taught by U.S.Pat. No. 3,398,371 Lamb, wherein is taught the use of a relay coil inseries with the magnetron, which coil is responsive to magnetron currentto control a set of relay contacts so as to by-pass a current-limitingserially connected resistor in the supply line to the magnetron powertransformer primary. U.S. Pat. No. 3,591,826 Valles teaches the use ofhot-wire relays connected across a power transformer low voltagesecondary winding used for cathode heating, wherein the relay contactsupon energization by-pass a currentlimiting series connected resistor inthe high voltage portion of the transformer secondary winding. U.S. Pat.

No. 3,651,371 Tingley teaches the use of a relay having its coilserially connected to a magnetron anode such that the relay isresponsive to magnetron current flowing through the relay coil to switchrelay contacts for by-passing a series resistance. When such relaycontacts are in the normally-open state, substantially all the currentsupplied to the magnetron is in the form of half-waves derived from thefirst half-wave voltagedoubler circuit used therein. When the loadcurrents imposed by the magnetron are such as to cause the relay to beenergized to close its contacts, the series limiting-resistor isby-passed, essentially allowingboth halves of the voltage doubler to beswitched into the circuit to effect the high power mode; in other words,applying full voltage to the magnetron tube. The aforementionedreferences in their various forms suffer from age applied to themagnetron by switching the input voltages to the voltage doublercircuit.

A reliable control arrangement is needed, therefore, for delaying theapplication of the full anode voltage to the magnetron tube and whichcontrol arrangement is less complex and less expensive of manufacturethan those presently available.

It is', therefore, an object of the present invention to provide acontrol arrangement for starting a magnetron, and more particularly, toprovide a two-step start circuit for a magnetron tube in a microwaveoven which is reliable and positive of operation and less ex pensive tomanufacture than those presently available.

SUMMARY OF THE INVENTION In accordance with one form of the presentinvention, a control arrangement for energizing a magnetron comprises afull-wave voltage multiplier having a pair of input terminals and a pairof putput terminals, one of the terminals being coupled with a firstline voltage terminal. A magnetron having an anode and a cathode iscoupled with the output terminals of the voltage multiplier. Relay meansresponsive to voltage to become energized at a predetermined levelthereof is coupled serially with said magnetron, the relay means havinga set of contacts, such that in the de-energized state of the relaymeans said contacts will serve to couple the second of said multiplierinput terminals to a neutral line terminal and in the energized state tocouple the second multiplier terminal to a second line voltage terminal. A delay means is coupled serially with said neutral terminal andsaid relay means contacts for providing a time delay before said relaymeans becomes energized, thereby allowing the magnetron to warm upbefore full voltage is applied thereto.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing there isshown a schematic representation of the preferred embodiment of thecontrol arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis illustrated schematically, a control arrangement for a magnetronoscillation tube such as is used in a microwave oven. Supplied byterminals 10, 12 and 114 denoted first line voltage, neutral and secondline voltage terminals respectively, the circuit, connected to the usual240 volts AC source, feeds a magnetron tube 20 having an anode 22, acathode 26 and a cathode heating filament 30. The filament 30 may besupplied from a transformer (not shown).

For the production'of the high voltage necessary to operate themagnetron tube, a voltage multiplier such as full-wave voltage doubler40 is provided comprising a pair of diodes 42 and 44 and a pair ofcapacitors 48 and 50. Input terminal 54 of the doubler 40 is coupled tofirst line voltage terminal through a current limiting resistor 64.Coupled across the output terminals 58 and 60 of voltage doubler 40 arethe anode 22 and the cathode 26 of magnetron 20, respectively. Coupledserially with mode 22 and output terminal 58 is field coil 70,. thepurpose of which is to provide the necessary magnetic fields for theoperation of magnetron tube 20.

In accordance with the present invention, relay means, such as a relayswitch 80 comprising a coil 82 and a switch 84, is provided forswitching the input voltage to doubler 40. Coil 82 is coupled inparallel with field coil 70 and is responsive to the voltage thereacrossto become energized at a predetermined level of that voltage. Switch 84comprises a set of contacts 86 and 88 and a switch arm 90 such that, inthe deenergized state of relay coil 82, switch arm 90 will be incoupling arrangement with contact 86. Coupled serially between contact86 and neutral terminal I2 is delay means, such as thermistor 94 havinga negative temperature coefficient, the thermistor 94 serving to delaythe application of full power to the magnetron to allow for warm-upthereof. Contact 88 is coupled with second line voltage terminal 14.

As is shown in the drawing, in the starting condition, voltage doubler40 has its input terminals 54 and 56 coupled to terminals 10 and 12respectively, and, as a result, approximately 118 volts AC is appliedthereto. However, since thermistor 94 is connected serially betweeninput 56 and neutral terminal 12, there is a relatively large voltagedrop thereacross, with a resultant lowered output of voltage doubler 40across output terminals 58 and 60. With the application of power toterminals 10, 12 and 14, it may be assumed that power is likewiseappliedto filament 30; in other words, filament 30 is activated at the sametime that voltage is applied to the magnetron anode 22 and cathode 26.As cathode 26 begins to heat, current is begun to be drawn throughmagnetron 20 and, likewise, through thermistor 94 and as current isdrawn therethrough, the effect is to cause the gradual heating up ofthermistor 94 which in turn causes the resistance thereof to fall. Asthis happens, the voltage drop across thermistor 94 also falls and as afurther result, the input voltage to voltage doubler 40 rises, followedby a higher voltage output from the doubler 40 to be applied tomagnetron 20.

There is a point then, for which the voltage output of the voltagedoubler, and as a result, the voltage across field coil 70, reaches sucha value that relay coil 82 becomes energized. With the energization ofcoil 82, the switch 84 is activated to switch arm 90 over to couple withcontact 88. Now the full line voltage from terminals 10 and 14 is causedto be applied to the input terminals-54 and 56 of voltage doubler 40.There is a resultant full wave ouput at terminals 58 and 60 of thevoltage doubler 40 reaching the 500-600 volt DC area.

At this point, the magnetron tube 20 has had 'sufficient time to warm upthoroughly, and the application of the approximately 600 volts DC placesthe magnetron in a full operating condition ready to deliver thenecessary microwave energy to the microwave oven interior.

Following are listed values for the several components which have beenused to place the present embodiment of this invention in operationalcondition:

Resistor 64 0.75 ohms [3.2 to 16.0 volts DC pull-in 400 ohms at roomtemperature 365 uf Relay coil 82 Thermistor 94 Capacitors 48, S0

It should be understood that such a control arrangement for a magnetronwill function properly without the field coils as discussed herein. Inother words, a permanent magnet or magnets may be employed to producethe necessary magnetic fields for the operation of the magnetron. Insuch a situation, the relay coil will become energized to effect pull-inupon the reaching of a predetermined level of current flow therethrough.It has also been considered that the thermistor may be replaced by aresistive heating element of an appropriate value, and coupled inparallel therewith and responsive to the heat generated thereby, anormally-open bimetal switch. Upon the drawing of a predetermined amountof current through the magnetron tube, and as a result through theheating element, the bimetal switch will close, effectively by-passingthe resistive heating element to allow the approximately 1 18 volts ACline voltage to be applied to the inputs of the voltage doubler.

It is to be further understood that the relay may be supplied withadditional sets of poles or switches to perform other'circuit functions.For example, one pole might be used to cause switching to effect reducedmagnetron filament voltage, and another might be used to turn on a timeror a light.

It should be apparent to those skilled in the art that the embodimentdescribed heretofore is considered to be the presently preferred form ofthis invention. In accordance with the Patent Statutes, changes may bemade in the disclosed apparatus and the manner in which it is usedwithout actually departing from the true spirit and scope of thisinvention.

What is claimed is:

l. A control arrangement for energizing a magnetron comprising:

a neutral terminal and a pair of line voltage terminals for supplyigelectric power;

a full-wave voltage multiplier having a pair of input terminals and apair of output terminals, one of the input terminals being coupled witha first of said line voltage terminals;

a magnetron having an anode and a cathode coupled respectively with theoutput terminals of said multiplier;

relay means responsive to voltage to become energized at a predeterminedlevel thereof, said relay means being coupled serially with saidmagnetron and further, having a set of contacts, in the deenergizedstate of the relay means to couple the second of said multiplier inputterminals to said neutral terminal, and in the energized state to couplesaid second multiplier terminal to said second line tenninal; and

delay means coupled serially with said neutral terminal and said relaymeans contacts for allowing a time delay before said relay means becomesenergized, thereby allowing the magnetron to warm up before full voltageis applied thereto.

2. The control arrangement of claim 1 wherein a field coil is coupled inparallel with said relay means for providing the necessary magneticfield for the operation of said magnetron tube.

3. The control arrangement of claim 1 wherein the delay means is athermistor.

4. The control arrangement of claim 1 wherein the delay means is aheating element and coupled in parallel therewith and responsive theretois a normally-open bimetal switch.

5. In a microwave oven, a control arrangement for energizing a magnetroncomprising:

a neutral terminal and first and second line voltage terminals forsupplying electric power;

a voltage multiplier having a pair of input terminals and a pair ofoutput terminals, one of said input terminals being coupled with saidfirst line voltage terminals;

a magnetron having an anode and a cathode coupled respectively with theoutput terminals of said multiplier;

a field coil coupled serially with said magnetron across said multiplieroutput terminals;

a relay switch having a set of contacts and further having a coilcoupled in parallel with said field coil, the switch being responsive tothe voltage across said field coil to become energized at apredetermined voltage level, in the de-energized state of the relay tocouple the second multiplier input terminal to said neutral terminal,and in the energized state to couple said second multiplier terminal tothe second line terminal; and

a thermistor coupled serially with said neutral terminal and said switchcontacts for allowing a time delay before said switch becomes energizedto effect gradual warm-up of said magnetron.

6. In a microwave oven, a control arrangement for energizing amagnetron, said arrangement comprising:

a neutral terminal and first and second line voltage terminals forcoupling to a source of electric power;

a full-wave voltage doubler having a pair of input terminals and a pairof output terminals, one of said input terminals being coupled with saidfirst line voltage terminal;

a current limiting resistor coupled serially with said first lineterminal;

a magnetron having an anode and a cathode coupled serially with theoutput terminals of said doubler;

a field coil coupled serially with said magnetron across said doubleroutput terminals;

a relay having a coil and a set of normally-open contacts, the coilbeing coupled in parallel with said field coil and responsive to thevoltage thereacross-to become energized at a predetermined voltagelevel, in the de-energized state of the relay to couple the seconddoubler input terminal to said neutral terminal, and in the energizedstate to couple said second doubler input terminal to the second lineterminal;

and a thermistor coupled serially with said neutral terminal forallowing a time delay before said relay becomes energized for effectinggradual warm-up

1. A control arrangement for energizing a magnetron comprising: aneutral terminal and a pair of line voltage terminals for supplyigelectric power; a full-wave voltage multiplier having a pair of inputterminals and a pair of output terminals, one of the input terminalsbeing coupled with a first of said line voltage terminals; a magnetronhaving an anode and a cathode coupled respectively with the outputterminals of said multiplier; relay means responsive to voltage tobecome energized at a predetermined level thereof, said relay meansbeing coupled serially with said magnetron and further, having a set ofcontacts, in the de-energized state of the relay means to couple thesecond of said multiplier input terminals to said neutral terminal, andin the energized state to couple said second multiplier terminal to saidsecond line terminal; and delay means coupled serially with said neutralterminal and said relay means contacts for allowing a time delay beforesaid relay means becomes energized, thereby allowing the magnetron towarm up before full voltage is applied thereto.
 2. The controlarrangement of claim 1 wherein a field coil is coupled in parallel withsaid relay means for providing the necessary magnetic field for theoperation of said magnetron tube.
 3. The control arrangement of claim 1wherein the delay means is a thermistor.
 4. The control arrangement ofclaim 1 wherein the delay means is a heating element and coupled inparallel therewith and responsive thereto is a normally-open bimetalswitch.
 5. In a microwave oven, a control arrangement for energizing amagnetron comprising: a neutral terminal and first and second linevoltage terminals for supplying electric power; a voltage multiplierhaving a pair of input terminals and a pair of output terminals, one ofsaid input terminals being coupled with said first line voltageterminals; a magnetron having an anode and a cathode coupledrespectively with the output terminals of said multiplier; a field coilcoupled serially with said magnetron across said multiplier outputterminals; a relay switch having a set of contacts and further having acoil coupled in parallel with said field coil, the switch beingresponsive to the voltage across said field coil to become energized ata predetermined voltage level, in the de-energized state of the relay tocouple the second multiplier input terminal to said neutral terminal,and in the energized state to coUple said second multiplier terminal tothe second line terminal; and a thermistor coupled serially with saidneutral terminal and said switch contacts for allowing a time delaybefore said switch becomes energized to effect gradual warm-up of saidmagnetron.
 6. In a microwave oven, a control arrangement for energizinga magnetron, said arrangement comprising: a neutral terminal and firstand second line voltage terminals for coupling to a source of electricpower; a full-wave voltage doubler having a pair of input terminals anda pair of output terminals, one of said input terminals being coupledwith said first line voltage terminal; a current limiting resistorcoupled serially with said first line terminal; a magnetron having ananode and a cathode coupled serially with the output terminals of saiddoubler; a field coil coupled serially with said magnetron across saiddoubler output terminals; a relay having a coil and a set ofnormally-open contacts, the coil being coupled in parallel with saidfield coil and responsive to the voltage thereacross to become energizedat a predetermined voltage level, in the de-energized state of the relayto couple the second doubler input terminal to said neutral terminal,and in the energized state to couple said second doubler input terminalto the second line terminal; and a thermistor coupled serially with saidneutral terminal for allowing a time delay before said relay becomesenergized for effecting gradual warm-up of said magnetron.